This invention relates to mobile antennas and more particularly to a dual polarized tri-band antenna for use in vehicles.
Currently there is a growing need for wireless mobile telephones. Common places on a vehicle for mounting mobile antennas include the roof, rain gutter, bumper, trunk lid, mirror bracket, fender and the side of the vehicle. The simplest mobile VHF/UHF antenna is the quarter wave vertical xe2x80x9cwhipxe2x80x9d antenna mounted on a high grade standoff insulator on the roof of a car. The metal body of the vehicle serves as a ground plane but can distort the normal circular radiation pattern of a vertical antenna.
At the center frequency of the citizen""s band (27.185 MHz) a quarter wave antenna is 108.62 inches (about 10 feet). Such an antenna can strike many overhead obstructions, causing it to bend and alter the angle of radiation when the vehicle is moving. Mounting a 10 foot antenna on the roof of a car is not feasible. An antenna that is physically shorter than a quarter wavelength must have inserted into it a suitable loading coil to bring its electrical length up to a quarter wave.
The performance of a mobile whip antenna can be improved by adding capacitance to the portion of the antenna above the loading coil. This capacitance tends to resonate with the inductance of the coil. Since the impedance of the whip antenna is lower than that of the coaxial line that brings power from the transmitter, an impedance matching network is needed.
Additionally, in general, an antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise transmission and/or reception can be impaired. For the strongest signals, the transmitting and receiving signals each should have the same polarization, either horizontal or vertical. Oftentimes communication must be between stations which use vertical polarization and horizontal polarization. Reflections/refractions due to buildings/land masses cause cross-polarization of signals. Polarization of satellite signals is circular.
It is a general object of this invention to provide an improved multiband antenna which is horizontally and vertically polarized and effective to transmit and receive in the broad frequency bands 140-170 MHz, 200-225 MHz and 400-480 MHz to include land mobile, HAM and satellite uses
Another object of this invention is the provision of a mobile antenna which is compact and of low height which permits mounting on the top of a vehicle.
Still another object of this invention is the provision of an antenna which does not require lossy band restrictive coils or windings (to bring the electrical length of the antenna up to a requisite wavelength) or tuning capacitors, thus providing an increase in signal strength.
Yet another object of this invention is the provision of a multi-element antenna system that significantly reduces flutter (picket-fencing) in the signal.
In accordance with the present invention there is provided a dual polarization antenna for receiving and transmitting high frequency (VHF/UHF) signals in conjunction with a substantially horizontal conducting vehicle panel defining a ground plane, said dual polarization antenna comprising:
a first, second and third radiative element each comprised of an electrically conductive material, each said radiative element being generally linear and extending between a proximal end and a distal end, and
means mountable of the vehicle panel for securing the proximal ends together at a common point and in electrical circuit relation with one another, the radiative elements extending vertically upwardly and outwardly from said common point whereby to form an imaginary cone with the proximal ends forming the apex of the cone,
said radiative elements each being of a different length and disposed at an angle relative to the ground plane to provide horizontal and vertical polarization and jointly resonate in a first, second and third frequency band.
In a preferred embodiment, the motor vehicle defines an electrical ground potential and the antenna is electrically insulated from said motor vehicle ground potential. The radiative elements forming the cone are disposed at an angle of between 15xc2x0 to 45xc2x0 relative to the geometric axis of the cone. and the distal ends are circumferentially spaced at 120xc2x0.
Preferably, the radiative elements forming the cone are disposed at an angle of about 60xc2x0 to the ground plane and resonate in the frequency bands of about 140-170 MHz, 200-225 MHz and 400-480 MHz; and the length of the first, second and third radiative element is, respectively, about 16xc2xe inches, 18xc2xd inches and 19 inches,
Advantageously, an antenna having the above construction eliminates xe2x80x9clossyxe2x80x9d coils, capacitors and matching structures and has high power handling capabilities (200+watts); achieves transceiving efficiency/gain in multiple frequency bands; provides broad frequency in each frequency band; reduces null/flutter problems; provides effective xe2x80x9cdualxe2x80x9d polarization radiation away from the horizon in addition to efficient xe2x80x9cnearxe2x80x9d horizon pattern; and provides extremely wide efficient continuous frequency receiving capabilities in a simple but compact construction.
The novel features of this invention are set forth with particularity in the appended claims. The invention itself will be best understood from the following description when read in connection with the accompanying drawings.